Method of making an optical fiber having voids

ABSTRACT

Microstructured optical fiber and method of making. Glass soot is deposited and then consolidated under conditions which are effective to trap a portion of the consolidation gases in the glass to thereby produce a non-periodic array of voids which may then be used to form a void containing cladding region in an optical fiber. Preferred void producing consolidation gases include nitrogen, argon, CO 2 , oxygen, chlorine, CF 4 , CO, SO 2  and mixtures thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims the benefit of priorityto U.S. patent application Ser. No. 11/583098, filed on Oct. 18, 2006now U.S. Pat. No. 7,450,806, the content of which is relied upon andincorporated herein by reference in its entirety, and also claims thebenefit of priority under 35 U.S.C. §119(e) of U.S. ProvisionalApplication Ser. No. 60/734,995 filed on Nov. 8, 2005, ProvisionalApplication Ser. No. 60/789,798 filed on Apr. 5, 2006, and ProvisionalApplication Ser. No. 60/845,927 filed on Sep. 20, 2006, the contents ofwhich are relied upon and incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to optical fibers, and morespecifically to microstructured optical fibers and methods for makingmicrostructured optical fibers.

2. Technical Background

Optical fibers formed of glass materials have been in commercial use formore than two decades. Although such optical fibers have represented aquantum leap forward in the field of telecommunications, work onalternative optical fiber designs continues. One promising type ofalternative optical fiber is the microstructured optical fiber, whichincludes holes or voids running longitudinally along the fiber axis. Theholes generally contain air or an inert gas, but may also contain othermaterials. The majority of microstructured fibers have a plurality ofholes located around the core, wherein the holes continue for arelatively long (e.g. for many tens of meters or more) distance alongthe length of the fiber, and typically the holes extend along the entirelength of the optical fiber. These cladding holes are also mosttypically arranged in a regular, periodic formation around the core ofthe optical fiber. In other words, if cross sections of the opticalfiber are taken along the length of the optical fiber, the sameindividual holes can be found in essentially the same periodic holestructure relative to one another. Examples of such microstructuredfibers include those described in U.S. Pat. No. 6,243,522.

Microstructured optical fibers may be designed to have a wide variety ofproperties, and may be used in a wide variety of applications. Forexample, microstructured optical fibers having a solid glass core and aplurality of holes disposed in the cladding region around the core havebeen constructed. The position and sizes of the holes may be designed toyield microstructured optical fibers with dispersions ranging anywherefrom large negative values to large positive values. Such fibers may beuseful, for example, in dispersion compensation. Solid-coremicrostructured optical fibers may also be designed to be single modedover a wide range of wavelengths. Most solid-core microstructuredoptical fibers guide light by a total internal reflection mechanism; thelow index of the holes acts to lower the effective index of the claddingregion in which they are disposed.

Micro-structured optical fibers are typically manufactured by theso-called “stack-and-draw” method, wherein an array of silica rodsand/or tubes are stacked in a close-packed arrangement to form apreform, that can be drawn into fiber using a conventional tower setup.There are several disadvantages to the stack and draw method. Theawkwardness of assembling hundreds of very thin canes (defined by rodsor tubes), as well as the possible presence of interstitial cavitieswhen stacking and drawing cylindrical canes, may affect dramatically thefiber attenuation by introducing soluble and particulate impurities,undesired interfaces and inducing a reshaping or deformation of thestarting holes. Moreover, the relatively low productivity and high costmake this method not much suitable for industrial production.

SUMMARY OF THE INVENTION

One aspect of the present invention relates to a method of making anoptical fiber comprising forming via chemical vapor deposition (CVD)operation a soot containing optical fiber preform. The soot preform isconsolidated in a gaseous atmosphere which surrounds the preform underconditions which are effective to trap a portion of the gaseousatmosphere in the preform during said consolidation step, therebyresulting in the formation of non-periodically distributed holes orvoids in the consolidated preform, each hole corresponding to a regionof at least one trapped consolidated gas within the consolidated glasspreform. The consolidated preform having holes therein is then used tomake an optical fiber. At least some of the holes formed in the opticalfiber preform during the consolidation step remain in the drawn opticalfiber. By designing the hole containing region to correspond to thecladding of the optical fiber, these resultant optical fiber can be madeto exhibit a core region with a first refractive index and a claddingregion having a second refractive index lower than that of the core, thelower refractive index at least partly due to the presence of the holesin the cladding. Alternatively or additional the methods disclosedherein can be used to provide a hole containing region within thecladding to thereby improve the bend performance of the optical fibers.For example, using the fiber designs and methods disclosed herein, it ispossible to create optical fiber which exhibits an increase inattenuation at 1550 nm when bent around a 10 mm mandrel which is lessthan 20 dB/turn, more preferably less than 15 dB/turn, and even morepreferably less than 10 dB/turn. Similarly, using the fiber designs andmethods disclosed herein, it is possible to create an optical fiberwhich exhibits an attenuation increases at 1550 nm of less than 3dB/turn, more preferably less than 1 dB/turn, even more preferably lessthan 0.5 dB/turn, and most preferably less than 0.25 dB/turn when bentaround a 20 mm diameter mandrel. The methods and fiber designs describedherein are useful for making both fibers that are single moded andfibers that are multimoded at 1550 nm.

Preferably, the voids are located substantially, and more preferablyentirely in the cladding of the fiber such that they surround the corein a void containing region, and the voids are preferably substantiallyabsent from the core region. In some preferred embodiments, the voidsare located in void containing regions which are spaced apart from thecore of the optical fiber. For example, a relatively thin (e.g. having aradial width less than 40 microns, and more preferably less than 30microns) ring of a void containing region can be spaced apart from thecore of the optical fiber, but not extending entirely to the outerperimeter of the optical fiber. Spacing the void containing region apartfrom the core will assist in lowering the attenuation of the opticalfiber 1550 nm. Using a thin ring will facilitate making the opticalfiber single moded at 1550 nm. The optical fiber may or may not includegermania or fluorine to also adjust the refractive index of the core andor cladding of the optical fiber, but these dopants can also be avoidedand instead, the voids alone can be used to adjust the refractive indexof the cladding with respect to the core such that light is guided downthe core of the fiber. Using the consolidation techniques disclosed,optical fibers can be formed whose cross-sections exhibit a non-periodicdistribution of holes therein. By non-periodic distribution, we meanthat when one views a cross section of the optical fiber, the voids arerandomly or non-periodically distributed across a portion of the fiber.Cross sections taken at different points along the length of the fiberwill exhibit different cross-sectional hole patterns, i.e., variouscross sections will have slightly different randomly oriented holepatterns, distributions, and sizes. These holes are stretched(elongated) along the length (i.e. parallel to the longitudinal axis) ofthe optical fiber, but do not extend the entire length of the entirefiber. While not wishing to be bound by theory, it is believed that theholes extend less than a few meters, and in many cases less than 1 meteralong the length of the fiber.

Using the void producing consolidation techniques disclosed herein, itis possible to make optical fibers having cladding regions which exhibita total fiber void area percent (i.e., total cross-sectional area of thevoids divided by total cross-sectional area of the optical fiber×100)greater than 0.01 percent, more preferably greater than 0.025 percent,even more preferably greater than 0.05 percent, even more preferablygreater than about 0.1 percent and even more preferably greater thanabout 0.5 percent. Fibers have been made having total void area percentsgreater than about 1, and in fact even greater than about 5 or even 10percent. However, it is believed that, depending on fiber design, totalvoid area percent of less than 1, and even less than 0.7, would resultin greatly improved bend performance. In some preferred embodiments, thetotal void area percent in said fiber is less than 20, more preferablyless than 15, even more preferably less than 10, and most preferablyless than 5 percent. Such void containing cladding regions can be usedto lower the refractive index relative to the core and thus form thecladding region which guides light along the core of the optical fiber.By selecting the appropriate soot consolidation conditions, as will bedescribed below, a variety of useful optical fiber designs can beachieved. For example, by selecting the maximum void size in thecladding to be less than that of the wavelength of light which is to betransmitted (for example, less than 1550 nm for some telecommunicationssystems), and preferably less than one half of the wavelength of lightwhich is to be transmitted along the fiber, low attenuation fibers canbe achieved without having to use expensive dopants. Consequently, for avariety of applications, it is desirable for the holes to be formed suchthat at least greater than 95% of and preferably all of the holes in theoptical fiber exhibit a maximum hole size in the cladding for theoptical fiber which is less than 1550 nm, more preferably less than 775nm, most preferably less than about 390 nm. Likewise, it is preferablethat the mean diameter of the holes in the fiber be less than 7000 nm,more preferably less than 2000 nm, and even more preferably less than1550 nm, and most preferably less than 775 nm, all of which meandiameters are achievable using the methods disclosed herein. The fibersmade using the methods disclosed herein can achieve these mean diametersto within a standard deviation of 1000 nm, more preferably to within astandard deviation of 750 nm, and most preferably to within a standarddeviation of 500 nm. In some embodiments, the fibers disclosed hereinhave less than 5000 holes, in some embodiments less than 1000 holes, andin some embodiments the total number of holes is less than 500 holes ina given optical fiber perpendicular cross-section. Of course, the mostpreferred fibers will exhibit combinations of these characteristics.Thus, for example, one particularly preferred embodiment of opticalfiber would exhibit less than 200 holes in the optical fiber, the holeshaving a maximum diameter less than 1550 nm and a mean diameter lessthan 775 nm, although useful and bend resistant optical fibers can beachieved using larger and greater numbers of holes. The hole number,mean diameter, max diameter, and total void area percent of holes canall be calculated with the help of a scanning electron microscope at amagnification of about 800× and image analysis software, such asImagePro, which is available from Media Cybernetics, Inc. of SilverSpring, Md., USA.

Another aspect of the present invention relates to the microstructuredoptical fibers which can be made using the process described above. Onesuch microstructured optical fiber includes a core region having a firstrefractive index and a cladding region having a second refractive indexwhich is lower than that of the core region due at least partially tothe presence of the non-periodically distributed voids therein. Lightwhich is to be transmitted through the fiber is thereby retainedgenerally within the core. The voids preferably have a maximum diameterof 1550 nm or less and the resultant optical fiber exhibits anattenuation at least one wavelength between 600 and 1550 nm (mostpreferably the wavelength is 1550 nm) which is less than 500 dB/km, morepreferably less than 200 dB/km at 1550 nm. By “attenuation”, as usedherein and if not specifically designated as “multimode attenuation” or“single mode attenuation”, we mean the multimode attenuation of saidfiber if the fiber is multimoded at 1550 nm and the single modeattenuation if the fiber is single moded at 1550 nm. Using the voidproducing consolidation techniques disclosed herein, it is possible tomake optical fibers having cladding regions which exhibit a regionalvoid area percent greater than 0.5 percent, more preferably greater thanabout 1, even more preferably greater than about 5 and most preferablygreater than about 10 percent. In particular, it is possible to producesuch void containing cladding regions within a 10 micron distance of thecore of the optical fiber. While index of refraction adjusting dopantsmay be avoided using the techniques disclosed herein, preferably atleast one of germania or fluorine or a similar index of refractionadjusting dopant is employed together with the non-periodicallydistributed voids located in the cladding region of the optical fiber.However, use of germania and/or fluorine is not critical and, forexample, the fiber could if desired be entirely or substantially devoidof both germania and fluorine. As used herein, by non-periodicallydistributed, we mean that the voids or holes are non-periodic, i.e.,they are not periodically disposed within the fiber structure. While themethods of the present invention are not capable of periodic placementof each individual void with respect to other individual voids, as maybe the case with some other types of microstructured fibers, the methodsdisclosed herein are capable of enabling the placement of large or smallrelative amounts of voids at various locations within the radialdistribution of the optical fiber. For example, using the methodsdisclosed herein a higher regional void area percent of voids can beplaced in a region which is adjacent the core of the optical fibercompared to other regions in the fiber (e.g., in the core of the fiberor the outer cladding region of the optical fiber). Likewise, theaverage hole size and hole size distribution in the void containingregion can be controlled both in a radial and axial (i.e., along thelength) direction of the fiber. Consequently, a uniform non-periodicarray of holes can be located at a region in the fiber, and the relativevoid area percent and average hole size in this region is maintainedconsistently along the length of the fiber. While the fibers are notlimited to any particular diameter, preferably the outer diameter of thefiber is less than 775, more preferably less than 375, and mostpreferably less than 200 microns.

Such a fiber can be used in telecommunication networks (typically 850,1310 and 1550 nm windows) including long-haul, metro, access, premiseand data centers as well as data communication applications and controlarea networks within buildings and mobile (auto, bus, train, plane)applications (typically 600 to 1000 nm range). Such telecommunicationsnetworks typically include a transmitter and receiver which is opticallyconnected to the optical fiber. Consequently, for a variety ofapplications, it is desirable for the holes to be formed such that themaximum hole size in the cladding for the optical fiber is less than1550 nm, more preferably less than 775 nm, most preferably less thanabout 390 nm.

Such fibers can also be used as UV to IR light-pipes for medical,illumination, lithography and industrial applications. The cladding ofone preferred fiber comprises a plurality of non-periodicallydistributed void regions in the cladding, preferably located within a 10micron radial distance from the core, wherein such voids having amaximum diameter, as measured in the radial direction (cross-sectionperpendicular to the longitudinal fiber axis) of the fiber, of 1550 nmor less, more preferably 775 nm or less. The cladding of anotherpreferred fiber, comprises a plurality of non-periodically distributedvoid regions in the cladding, spaced from the core and within 20 micronsradial distance from the core, wherein such voids having a maximumdiameter, as measured in the radial direction of the fiber, of 1550 nmor less, more preferably 775 nm or less, most preferably less than about390 nm. The cladding of yet another preferred fiber, comprises aplurality of non-periodically distributed void regions in the cladding,within 40 microns radial distance from the outside edge of the core,wherein such voids having a maximum diameter, as measured in the radialdirection of the fiber, of 1550 nm or less, more preferably 775 nm orless, most preferably less than about 390 nm. The optical fibersdisclosed herein result in a number of advantages compared to variousoptical fibers known in the prior art. For example, the fibers disclosedherein are capable of superior bend resistance compared to fibers of theprior art while simultaneously exhibiting excellent mode fielddiameters. By superior, we mean that using the methods disclosed herein,it is possible to make fibers which are single moded at 1550 nm andcapable of less than 0.5 dB attenuation increase at 1550 nm per turn fora 20 mm diameter bend while simultaneously exhibiting mode fielddiameters greater than 10 microns, and more preferably greater than 11microns, at 1550 nm. Such excellent bend performance makes these fibersattractive candidates for fiber-to-the-home, access fiber,fiber-in-the-home applications, and fiber jumpers (these are typicallyshort sections of fiber (1-20 meters) with connectors on each end toconnect to an optical system or device). For example, the fibersdisclosed herein may be employed in an optical fiber telecommunicationssystem comprising a transmitter, a receiver, the fiber(s) beingoptically connected to said transmitter and receiver. Preferably in suchapplications (i.e. when the fiber is acting as a transmission fiber in atelecommunications system) the fiber is devoid of any active elementssuch as erbium, etc.

In addition, the fibers disclosed herein can be made to have highnumerical aperture (e.g. greater than 0.2, more preferably greater than0.4, and most preferably greater than 0.6 at 1550 nm), which willfacilitate their ability to be connected to other optical laser sourcesand increase the tolerance for fiber connectors. Such fibers are alsoexcellent candidates for automotive applications. In such applications,it is most preferable that the fiber exhibit a maximum void size of lessthan about 1550 nm, more preferably less than 775 nm, and mostpreferably less than about 390 nm.

The fibers disclosed herein can be made using a relatively low costmanufacturing process, because expensive dopants such as fluorine and/orgermania can be avoided, if desired, and the stack and drawmanufacturing process can likewise be avoided. The invention will alsoenable flexible dispersion control (positive, flat or negative), forexample, the achievement of large positive dispersion (>30 ps/nm/Km at1550 nm) for signal processing, or negative dispersion fibers (e.g.,<−200 ps/nm/Km at 1550 nm) which could be useful for dispersioncompensation. Alternatively, the methods disclosed herein can be usedsimply to add voids to a cladding of a fiber which is doped with one ormore of germania, phosphorous, aluminum, ytterbium, erbium, fluorine orother conventional fiber dopant materials, to increase the bendresistance thereof. In another alternative embodiment, the methodsdisclosed herein can be used to make a silica core (i.e., a fiber havingno germanium dopant in the core) fiber which has a cutoff wavelengthbelow 800 nm and a numerical aperture greater than about 0.08, morepreferably greater than about 0.10 at 1550.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention, and are incorporated into and constitute a part of thisspecification. The drawings illustrate various embodiments of theinvention, and together with the description serve to explain theprinciples and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an OVD method for forming a soot preform.

FIG. 2 illustrates a cross-sectional side view of a consolidationprocess in accordance with the invention.

FIG. 3 illustrates a redraw process for forming a core cane.

FIG. 4 illustrates consolidation of soot which has been deposited onto acore cane.

FIG. 5 illustrates the fully consolidated preform which results from theconsolidation step illustrated in FIG. 4.

FIG. 6 illustrates a photomicrograph of a fiber made in accordance withone embodiment of the invention.

FIGS. 7 and 8 together illustrate a rod in tube manufacturing processwhich may be employed with various methods of the present invention.

FIG. 9 illustrates a draw process and apparatus that maybe employed inthe method of the present invention.

FIG. 10 illustrates an SEM photomicrograph of a fiber made in accordancewith one embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The methods of the present invention utilizes preform consolidationconditions which are effective to result in a significant amount ofgases being trapped in the consolidated glass blank, thereby causing theformation of voids in the consolidated glass optical fiber preform.Rather than taking steps to remove these voids, the resultant preform isused to form an optical fiber with voids therein.

During the manufacture of transmission optical fibers by conventionalsoot deposition processes such as the outside vapor deposition (OVD)process or the vapor axial deposition (VAD) process, silica and dopedsilica particles are pyrogenically generated in a flame and deposited assoot. In the case of OVD, silica soot preforms are formed layer-by-layerby deposition of the particles on the outside of a cylindrical targetrod by traversing the soot-laden flame along the axis of the cylindricaltarget. Such porous soot preforms are subsequently treated with a dryingagent (e.g., chlorine) to remove water and metal impurities and are thenconsolidated or sintered inside a consolidation furnace into void-freeglass blanks at temperatures ranging from 1100-1500° C. Surface energydriven viscous flow sintering is the dominant mechanism of sintering,which results in densification and closing of the pores of the soot,thereby forming a consolidated glass preform. During the final stages ofsintering, the gases used in consolidation may become trapped as theopen pores are closed. If the solubility and permeability of the trappedgases in the glass are high at the sintering temperature, then the gasesare able to migrate through and out of the glass during theconsolidation process. Alternatively, gases which are still trappedafter the consolidation phase of the fiber manufacturing process may beoutgassed by holding the fiber preforms for a period until the gasesmigrate out through the glass preforms, thereby leaving one or morevoids with vacuum therein within the preform. During the draw operationwhen the optical fiber is drawn from the preform, these voids close,leaving a void-free or essentially void-free optical fiber. Inconsolidation processes which are employed to make conventionaltransmission optical fiber, the goal is to achieve an optical fiber thatis entirely free of voids in both the core and cladding region of theoptical fiber. Helium is often the gas utilized as the atmosphere duringthe consolidation of conventional optical fiber preforms. Because heliumis very permeable in glass, it very easily exits the soot preform andthe glass during the consolidation process, so that after consolidatingin helium the glass is free of pores or voids.

The present invention utilizes preform consolidation conditions whichare effective to result in a significant amount of gases being trappedin the consolidated glass blank, thereby causing the formation ofnon-periodically distributed voids in the consolidated glass opticalfiber preform. Rather than taking steps to remove these voids, theresultant preform is purposefully used to intentionally form an opticalfiber with voids therein, hi particular, by utilizing relatively lowpermeability gases and/or relatively high sintering rates, holes can betrapped in the consolidated glass during the consolidation process. Theterm sintered or consolidated glass, as used herein, refers to glassthat has gone through a soot consolidation step after a chemical vapordeposition soot deposition process such as the OVD or VAD processes.During the soot consolidation step, the soot goes through adensification process via exposure to high heat, thereby removing theopen porosity (i.e., voids or pores between the soot which is notsurrounded by densified glass) and leaving fully densified glass(although in the present invention some closed pores (i.e., voids orpores surrounded by fully densified glass) obviously remain. Such sootconsolidation step preferably takes place in a soot consolidationfurnace. The sintering rate can be increased by increasing the sinteringtemperature and/or increasing the downfeed rate of the soot preformthrough the sintering zone of the consolidation furnace. Under certainsintering conditions, it is possible to obtain glasses in which the areafraction of the trapped gases is a significant fraction of the totalarea or volume of the preform.

In one preferred embodiment of the invention, the non-periodicallydistributed holes or voids which are present in the optical fiber as aresult of using the processes disclosed herein are located in thecladding of the optical fiber. Such voids can be used to lowerrefractive index. By designing the consolidation parameters so that themaximum diameter of the holes or voids is less than the wavelength ofthe light which is to be transmitted along the length of the fiber (e.g.in the case of optical fibers for use in telecommunicationsapplications, less than 1550 nm), the fiber may be effectively used totransmit information at a particular wavelength.

FIG. 1 illustrates a method of manufacturing a soot optical fiberpreform 20 which can be used in accordance with the present invention.In the embodiment illustrated in FIG. 1, soot preform 2 is formed bydepositing silica-containing soot 22 onto an outside of a rotating andtranslating mandrel or bait rod 24. This process is known as the OVD oroutside vapor deposition process. Mandrel 24 is preferably tapered. Thesoot 22 is formed by providing a glass precursor 28 in gaseous form tothe flame 30 of a burner 26 to oxidize it. Fuel 32, such as methane(CH4), and combustion supporting gas 34, such as oxygen, are provided tothe burner 26 and ignited to form the flame 30. Mass flow controllers,labeled V, meter the appropriate amounts of suitable dopant compound 36silica glass precursor 28, fuel 32 and combustion supporting gas 34, allpreferably in gaseous form, to the burner 26. The glass former compounds28, 36 are oxidized in the flame 30 to form the generallycylindrically-shaped soot region 23. In particular, a dopant compound 36may be included if desired. For example, a germanium compound may beincluded as an index of refraction increasing dopant (e.g. in the coreof the fiber), or a fluorine containing compound may be included tolower the index of refraction (e.g. in the cladding and/or voidcontaining region of the fiber).

As illustrated in FIG. 2, the soot preform 20 including the cylindricalsoot region 23 may be consolidated in a consolidation furnace 29 to forma consolidated blank 31 (shown in subsequent FIG. 3). Prior toconsolidation, the mandrel 24 illustrated in FIG. 1 is removed to form ahollow, cylindrical soot blank preform. During the consolidationprocess, the soot preform 20 is suspended, for example, inside a purequartz muffle tube 27 of the consolidation furnace 29 by a holdingmechanism 21. Preferably, before the consolidation step the preform 20is exposed to a drying atmosphere. For example, a suitable dryingatmosphere may include about 95 percent to 99 percent helium and 1percent to 5 percent chlorine gas at a temperature of between about 950°C. and 1250° C. and a suitable drying time ranges from about 0.5 and 4.0hours. The soot preform can also be doped, if desired, for example usinga dopant gas having fluorine or other optical fiber dopants therein. Forexample, to dope with fluorine, SiF₄ and/or CF₄ gas maybe employed. Suchdopant gases maybe employed using conventional doping temperatures, forexample between about 950 and 1250° C. for 0.25 to 4 hours.

During the consolidation step, which preferably takes place after a sootdrying step, the furnace temperature is raised and the preform 20 isconsolidated at a suitable temperature, for example between about 1390°C. and 1535° C. to form a consolidated preform. Alternatively, gradientsintering may be employed whereby the soot preform 20 is driven downthrough a hot zone of the furnace 29 which is maintained at atemperature of between about 1225° C. to 1550° C., more preferablybetween about 1390° C. and 1535° C. For example, the preform maybe heldin an isothermal zone which is maintained at a desired dryingtemperature (950-4250° C.), after which the soot preform is driventhrough a zone which is maintained at a desired consolidationtemperature (e.g. 1225° C. to 1550° C., more preferably 1390° C. and1535° C.) at a rate of speed which is sufficient to result in thepreform 20 temperature increasing by greater than 1° C./min. Upper zonesof the furnace can be maintained at lower temperatures which facilitatea drying and impurity removal step. The lower zone can be maintained atthe higher temperatures desired for consolidation. In one preferredembodiment, the soot containing preform is downfed through aconsolidation hot zone at a first downfeed rate, followed by downfeedingof the preform through a second hot zone at a second downfeed rate whichis less than that of the first downfeed rate. Such a consolidationtechnique results in the outside portion of the soot preform sinteringbefore the rest of the preform sinters, thereby facilitating trapping ofgases which will in turn facilitate formation of and retaining of voidsin the resultant consolidated glass. For example, the preform can beexposed to such suitable consolidation temperatures (e.g. greater thanabout 1390° C.) at a first speed which is sufficient to result in thepreform temperature increasing by more than 15° C./min, more preferablygreater than 17° C./min, followed by at least a second downfeedrate/consolidation temperature combination which is sufficient to resultin the preform heating by at least about 12° C./mm, more preferablygreater than 14° C./min. Preferably, the first consolidation rateresults in the outside of the preform increasing in temperature at arate which is greater than 2, more preferably greater than 10, even morepreferably greater than about 20, and most preferably greater than 50°C. /min higher than the heating rate of the second consolidation rate.If desired, a third consolidation step or even 5 or more additionalconsolidation steps can be employed which heats at a slower rate (e.g.less than 10° C./min). Alternatively, the soot preform can be sinteredat even faster rates in order to create more voids by driving the sootpreform through a furnace hot zone where the temperature is greater than1550° C., more preferably greater than 1700° C., even more preferablygreater than 1900° C. Alternatively, the soot preform can be sintered ateven faster rates external to the furnace by using an open flame orplasma torch in contact with the soot.

Preferred sintering gases (i.e., the gas that surrounds the preformduring the sintering step) which may be used in the consolidation stepare those which comprise at least one gas selected from the groupconsisting of nitrogen, argon, CO₂, oxygen, chlorine, CF4, CO, SO2,krypton and mixtures thereof. Each of these gases exhibits a relativelylow permeability in silica glass at or below the consolidationtemperature which is suitable for forming voids in accordance with themethods present invention. Preferably these void producing gases areemployed either alone or in combination in an amount between 5 and 100percent by volume, more preferably between about 20-100 percent byvolume and most preferably between about 40-100 percent by volume. Theremainder of the sintering gas atmosphere is made up of a suitablediluent or carrier gas such as, for example, helium, hydrogen,deuterium, or mixtures thereof. It some of the embodiments describedherein, e.g. when additional soot is planned to be deposited via OVD tothe resultant glass perform or cane subsequent to the void producingconsolidation process, it is preferable to utilize a sintering gas whichemploys less than 10 percent oxygen, more preferably less than 5 percentoxygen, and most preferably essentially no oxygen, otherwise due toexposure to hydrogen formed in the OVD process, some seeds might belost. Generally speaking, the greater the percentage by volume of voidproducing gases (nitrogen, Ar, CO₂, O₂, Cl₂, CF4, CO, SO2, krypton, ormixtures thereof) that is employed in the sintering gas, the larger andmore abundant the voids will be in the resultant consolidated glass.More preferably, the sintering gas for use in forming the voids duringthe consolidation step comprises at least one gas selected from thegroup consisting of nitrogen, argon, CO₂, oxygen, and krypton, andmixtures thereof. These gases can be utilized entirely alone or inmixtures of such gases along with a carrier gas such as helium. Oneparticularly preferred void producing gas is nitrogen. Applicants havefound when employing nitrogen and/or argon either together orindividually as a void producing sintering gas it is preferred that thenitrogen and/or argon be employed in the sintering atmosphere in anamount greater that 10 percent by volume, more preferably greater than30 percent by volume, even more preferably greater than about 50 percentby volume, and most preferably greater than about 65 percent by volume,with the remainder of the sintering atmosphere being a carrier gas suchas helium. These gases have been successfully employed at concentrationsgreater than 85 percent by volume. In fact, up to 100 percent nitrogengas, up to 100 percent argon gas, and up to 100 percent oxygen gas havebeen utilized successfully. Voids can also be created by sintering thesoot in a low permeability gas (e.g. nitrogen, argon, CO₂, oxygen,Chlorine, CF4, CO, SO2) under a partial vacuum (e.g., wherein thepreform is immersed in a sintering atmosphere is at a pressure ofbetween about 40 to 750 Torr). Using the void producing consolidationtechniques disclosed herein, it is possible to make optical fibershaving a cladding comprises a void containing region having a voidregional void area percent greater than 0.5 percent, more preferablygreater than about 1, even more preferably greater than about 5 and mostpreferably greater than about 10 percent. Regional void area percent, asused herein, means the total area of the voids in a void containingregion divided by the total area of the void containing region (when theoptical fiber is viewed in cross-section taken perpendicular to the axisof the optical fiber) times 100, the void containing region beingdefined by the inner and outer boundaries of the void containing region.For example, if the inner edge of the innermost void in the fiber has aradial location of 4 microns from the axial centerline of the fiber, andthe outer edge of the outer most void in the fiber has a radial locationof 60 microns from the centerline, then the area of the void containingregion is approximately 11309−50=11259 square microns. If the totalcross sectional area of voids contained in this void containing regionis 1100 square microns, then the void area percent of the voidcontaining region is approximately 9.8 percent.

Using the preferred sintering gases described above, it is desirable toemploy a consolidation process which includes a downfeed of the preformat a rate and temperature which is sufficient to result in at least someof the consolidation gases being intentionally trapped. This can occur,for example, by heating of at least a portion of the soot preformgreater than about 10° C./min, more preferably greater than about 12°C./min, even more preferably greater than about 14° C./min. Thesintering temperatures employed in the present invention preferably aregreater than 1100° C., more preferably greater than 1300° C., even morepreferably greater than 1400° C., and most preferably greater than 1450°C. One particularly preferred sintering temperature is approximately1490° C.

FIG. 3 illustrates a process which may be used to draw a core cane foruse in the present invention. For example in one such embodiment, a sootpreform is formed as described above with respect to FIG. 1, after whichthe soot preform is consolidated using conventional consolidationtechniques (e.g., using consolidation temperatures of higher than 1300°C. in an atmosphere of 100 percent helium) to form a void free corepreform. For example, in the case of a fiber preform which is to be usedto make a pure silica core fiber, the core preform would consist ofrelatively pure silica with no significant index of refraction adjustingdopants. Alternatively, in the case of an optical fiber preform which isto be used to make a pure germania doped core fiber, the core preformcould consist of a germania doped core region and optionally a portionof the cladding (e.g. undoped silica cladding). The resultantconsolidated core blank 31 is placed in a core cane draw furnace 37 andat least one rod-shaped core cane segment 33 having a reduced outerdiameter is drawn therefrom. The preform blank 31 is heated to atemperature of for example, between about 1700° C. and 2000° C. Thecontrols 38 control the tension applied to the cane by suitable controlsignals to a tension mechanism 40, shown here as two tractor wheels, todraw down the cane 33 at the proper speed. In this way, it is possibleto derive a length of core cane 33 having an outer diameter dimension offor example between about 1 mm and 16 mm. This core cane can then beused as the target or mandrel 24 for additional soot deposition or asthe rod in a rod in tube process, as will be described further below.

In one preferred embodiment, the process described above with respect toFIG. 3 can be used to form a core cane blank which can then be used asthe target or mandrel for additional soot deposition which will beconsolidated using the void forming techniques disclosed herein and thuseventually become the cladding of the optical fiber. In one suchembodiment, for example, a fully consolidated, void free glass core canecan be used as the bait rod 24 in the soot deposition step illustratedin FIG. 1. The glass core cane may be undoped silica so the resultantoptical fiber will be silica core fiber whose core consists essentiallyof pure silica. Alternatively, the core cane may consist of one or moredoped regions which together form the light transmitting core region ofthe optical fiber. After the soot is deposited onto the glass core cane,the outer soot region 120 can be fully consolidated in consolidationfurnace 129 as illustrated in FIG. 4. Preferably, during thisconsolidation step, the void forming consolidation process describedabove is carried out to form a void containing consolidated opticalfiber preform 150, as illustrated in FIG. 5.

As described above, preferred gases for use in the void formingconsolidation step include at least one gas selected from the groupconsisting of nitrogen, argon, CO₂, oxygen, Chlorine, CF4, CO, SO2,krypton, and mixtures thereof. Preferably these void producing gases areemployed either alone or in combination in an amount between 5 and 100percent by volume, more preferably between about 20-100 percent byvolume and most preferably between about 40 and 100 percent by volume.The remainder of the sintering gas atmosphere is made up of a suitablediluent or carrier gas such as, for example, helium, hydrogen,deuterium, or mixtures thereof. Generally speaking, the greater thepercentage of void producing gases (nitrogen, Ar, CO₂, Kr, O₂, Cl₂, CF4,CO, SO2) employed in the sintering gas, the larger and more abundant thevoids will be in the resultant consolidated glass. One particularlypreferred void producing gas is nitrogen, which is preferably employedin an amount greater that 10 percent by volume, more preferably greaterthan 30 percent by volume, even more preferably greater than about 50percent by volume, and most preferably greater than about 65 percent byvolume, with the remainder of the sintering atmosphere being a carriergas such as, for example, helium. Voids can also be created by sinteringthe soot in a low permeability diluent gas (e.g. nitrogen, argon, CO₂,oxygen, Chlorine, CF4, CO, SO2) under a partial vacuum (e.g., whereinthe sintering atmosphere is at a pressure of between about 40 to 750Torr), and in such cases use of a diluent relatively high permeabilitygas such as helium is not necessary. Using the void producingconsolidation techniques disclosed herein, it is possible to makeoptical fibers whose cladding comprises a void containing region havinga regional void area percent greater than 0.5 percent, more preferablygreater than about 1 percent. It is even possible using these techniquesto achieve greater than about 5 and even greater than about 10 percentregional void area percent. The regional void area percent is preferablyless than 50 percent, more preferably less than 20 percent. Mostpreferably, the region having holes does not extend to the outer edge ofthe cladding such that there are open voids or holes on the outside ofthe fiber.

The sintering temperatures employed in the present invention preferablyrange from 1100° C. to 1550° C., more preferably between 1300° C. and1500° C., and most preferably between 1350° C. and 1500° C. Onepreferred sintering temperature is approximately 1490° C. The gaseousatmosphere employed during the consolidation process, the temperatureinside the consolidation furnace, and preform consolidation rate areselected so that, during the soot consolidation process, gases areintentionally trapped within the preform, forming holes in theconsolidated glass. These gas containing voids are preferably notentirely outgassed prior to and/or during the fiber draw process, sothat the voids remain in the fiber after the fiber has been drawn. Avariety of process parameters can be controlled to vary and control thesize of the voids. For example, increasing the consolidation time ortemperature can increase the void size, as the increased temperaturecauses the gases trapped within the voids to expand. Similarly, the sizeand area percent of the voids can be impacted by the draw conditions.For example, a longer hot zone in a draw furnace and/or faster drawspeeds tend to increase the size as well as the area percent of theholes. Selection of a gas that is more permeable in glass at theconsolidation temperature will result in smaller voids. Sintering ratecan also have a significant effect on hole size and hole quantity. Afaster sintering rate will result in the formation of more and largervoids. However, use of sintering rates that are too slow will result inno voids being formed, as the gas will have time to escape through theglass. Consequently, the downfeed rate of the preform and/or theconsolidation temperature employed are preferably high enough to resultin the heating of at least a portion of the preform at a rate greaterthan about 10° C./min, more preferably greater than about 12° C./min,even more preferably greater than about 14° C./min. Generally speaking,an optical fiber preform having a lower soot density will result information of more voids. However, the density of the deposited soot in aparticular optical fiber preform can be varied to position more holes(higher regional void area percent) where desired. For example, a firsthigh density soot region can be deposited directly onto a consolidatedglass (e.g. pure silica) core cane, followed by a second region of soothaving a lower density than that of the first. We have found that thiscauses a higher void area percent to form near the core (i.e. in thehigh density soot region). The silica containing soot preferably has abulk density of between about 0.10 g/cc and 1.7 g/cc, more preferablybetween about 0.30 g/cc and 1.0 g/cc. This effect can also be used toform consolidated void containing preforms which alternate between lowor no void containing regions and higher void containing regions;wherein the initial soot density radial variation is greater than 3percent over a distance of at least 100 microns. Such preforms can beused, for example, to make optical fibers having cladding regions whichalternate between regions of void free glass and void containing glass.Fibers having such alternating void containing and void-free regionswould exhibit properties useful as Bragg gratings.

Referring to FIG. 5, using the techniques described above, an opticalfiber preform 150 can be formed which comprises a void-free core region151 which is surrounded by a cladding 152 which is comprised of aplurality of voids. By forming the void containing region in cladding152 with a sufficient number of voids of an appropriate size, cladding152 will serve as an optical cladding which guides light along coreregion 151 after the optical fiber preform is drawn into an opticalfiber. Alternatively, the void containing region can be employed toimprove the bend performance of the optical fiber. If desired, prior todrawing the preform 150 into an optical fiber, additional soot can bedeposited over cladding region 152 and consolidated. The additionaldeposited cladding material may or may not be consolidated to containvoids, as desired.

An example of a fiber which is drawn from such a preform is illustratedin FIG. 6. The fiber is FIG. 6 comprises a pure silica core region whichis surrounded by a cladding region which comprises voids which arepositioned to be effective to guide light along the silica core. Thefundamental mode of the fiber of FIG. 6 exhibited an attenuation of 0.28dB/km at 1550 nm, even though this fiber was made using relatively crudeexperimental manufacturing equipment. However, using more suitableequipment, attenuations of less than 0.2 dB/km at 1550 nm are certainlypossible.

Alternatively, instead of depositing soot onto an already formed corecane, the void forming process described above can be used to form atube of consolidated glass having a void containing region therein asdescribed above with respect to FIG. 2, and that tube can be used tosleeve a core cane. For example, the above described process can be usedto form a soot preform onto a removable mandrel 24, after which time themandrel is removed and the soot preform is consolidated as describedabove to form a consolidated glass tube having voids therein. Theresultant tube 65 which contains voids therein can be used to sleeve acore cane 35. Such sleeving can be accomplished, for example, usingconventional rod in tube manufacturing techniques, as illustrated inFIGS. 7 and 8. In FIG. 7, pure (i.e., substantially free of index ofrefraction increasing dopants such as germanium) silica core cane 35 isinserted into void containing cladding sleeve portion 65, althoughalternatively the core region or the cladding could be doped withconventional index adjusting dopants such as germanium or fluorine. InFIG. 8, core cane 35 and cladding sleeve portion 65 are heated to asuitable temperature (e.g., greater than about 1300 to 1500° C.) andthen redrawn to a smaller diameter using well known rod in tubemanufacturing process steps, thereby forming an optical fiber preformfrom which can be drawn an optical fiber having a pure silica coreregion surrounded by a void containing cladding region in accordancewith the invention.

In any of the embodiments disclosed herein, the resulting finalconsolidated optical fiber preform 50 may be drawn into an optical fiberby positioning the preform within a draw furnace 52 as shown in FIG. 9,and then heating and drawing the optical fiber 54 using conventionalmethods and apparatus. The fiber 54 is then cooled in cooling chamber 55and measured for final diameter by non-contact sensor 56, One or morecoatings may be applied and cured by coating apparatus 58, as is alsoconventional. During draw, the fiber 54 passes through a tensionassembly 60 whereby tension is applied to draw the fiber 54 from thepreform 50. The tension is controlled via control apparatus 61 tomaintain the fiber diameter at a predetermined set point. Finally, thecoated fiber 54 is wound by feedhead 62 onto a fiber storage spool 64.

The same process described above with respect to FIG. 3 for forming corecanes can alternatively be used to redraw void containing consolidatedtubes. Such a redraw process can be used to modify the size of the voidscontained in the tube. For example, the greater the diameter reductionthat occurs when the void containing perform is redrawn, the smaller thevoid size will be in that preform.

Using the void producing consolidation techniques disclosed herein,optical fibers have been achieved which are comprised of a core regionhaving a first refractive index and a cladding region having a secondrefractive index lower than that of the core such that light which istransmitted through the fiber is retained generally within the core,whereby said voids are located in and thereby form the cladding of saidoptical fiber and the void area percent of the voids is substantiallynon-zero.

Using the techniques described herein, fibers can be made wherein themaximum size of any of the voids, in the region where the fraction ofpower of light is greater than 80 percent, is less than the wavelengthof light being transmitted for applications relating totelecommunications automotive applications. By maximum size, we mean themaximum diameter of any particular void when the optical fiber is viewedalong the axial direction of the fiber in perpendicular cross-section.For example, fibers have been made wherein the maximum size of all ofsaid voids, in the region where the fraction of power of light isgreater than 80 percent, and even more preferably in the region wherethe fraction of power of light is greater than 90 percent, is less than5 microns, more preferably less than 2 microns, even more preferablyless than 1 micron, and most preferably less than 0.5 microns.

Using the techniques described herein, fibers can be made having voidcontaining regions which exhibit regional void area percents of greaterthan 1 percent, more preferably greater than 10 percent, and mostpreferably greater than 30 percent.

The process described above has been generally limited to making silicacore optical fiber, i.e., fibers having a relatively pure silica coreregion surrounded by a void containing cladding region. Alternatively,index adjusting dopants such as germanium and fluorine can be used, ifdesired, either individually or together, to further adjust therefractive index of the core with respect to the index of refraction ofthe cladding. For example, in one such preferred embodiment, a germaniumcore cane is used as a starter rod, upon which additional soot claddingmaterial is deposited, preferably using OVD deposition techniques asdescribed above. The soot cladding region is then consolidated asdescribed above to form a void containing cladding region around thegermania doped silica core region. In another alternative embodimentinvolving index adjusting dopants, a silica core cane is employed as thestarter rod for a soot cladding region. However, during the voidproducing consolidation step, in addition to the void producing dopantgas, a fluorine dopant source is provided (e.g. SiF4 gas) tosimultaneously dope the void containing region with fluorine. In thisway, a fluorine doped void containing region can be formed around asilica core region.

EXAMPLES

The invention will be further illustrated by the following examples.

Step 1—Core cane preparation: 8 and 15 mm diameter pure silica corecanes were made via standard OVD processing. SiO2 soot (0.5 g/ccdensity) was first deposited onto a removable bait rod, then the baitrod was removed and the resultant soot was consolidated using standardconsolidation (2 hour dry in helium +3 percent chlorine @1000° C.)followed by down driving through a hot zone set at 1500° C. at 6mm/minute downfeed rate (corresponding to 3° C./min heat up rate) in aHe only atmosphere in order to sinter the soot into clear void-freeconsolidated glass blank. The blank was then redrawn at 1900° C. underless than 500 Torr (partial vacuum) to the centerline to close thecenterline hole and result in void free consolidated silica core caneshaving a diameter of 8 mm or 15 mm. Unless otherwise noted, in each ofthe examples below, when the fiber was drawn the fiber was coated usingconventional coatings (i.e. conventional acrylate based primary andsecondary coatings).

Example 1

3000 grams of SO₂ (0.48 g/cc density) soot were deposited using anoutside vapor deposition process to form an SiO₂ soot blank, i.e., bydepositing onto a 1 meter long×10 mm diameter removable alumina baitrod. The alumina bait rod was removed and an 8 mm diameter core caneconsisting of pure (undoped) consolidated silica was inserted into theSiO2 soot blank. This rod in soot assembly was then sintered as follows.The assembly was first dried for 2 hours in an atmosphere of 97 percenthelium and 3 percent chlorine at 1000° C. followed by down driving at 32mm/min (resulting in an increase in temperatures of the preform ofapproximately 16° C./min.) through a hot zone set at 1500° C. in a 100percent nitrogen sintering atmosphere. The preform assembly was thanre-down driven (i.e., a second time) through the hot zone at 25 mm/min(approximately 12.5° C./min. heat up rate of the preform), then finalsintered at 6 mm/min (approximately 3° C./min heat up rate) in order tosinter the soot into a nitrogen-seeded overclad blank. The first higherdownfeed rate was employed to glaze the outside of the optical fiberpreform, which facilitates trapping of the gases in the preform. Theblank was then placed for 24 hours in an argon purged holding oven setat 1000° C.

The resultant optical fiber preform was drawn into a 125 micron diameteroptical fiber at 1 m/s using a draw furnace having a hot zone of about2.54 cm length and set at 2100° C. SEM analysis (FIG. 6) of the end faceof a cross-section of the resultant optical fiber showed about 22 microndiameter solid silica core and a cladding containing 3.5 regional voidarea percent (area of holes divided by area of the hole containingregion×100)with an average diameter of 0.3 microns (300 nm) and amaximum hole diameter of 0.50 microns (500 nm) with a standard deviationof 0.08 microns, and comprising approximately 3400 holes, resulting inabout 7900 total number of holes across the entire fiber cross-section.The fiber's total void area percent (area of the holes divided by totalarea of the optical fiber cross-section×100) was about 3.4 percent.Optical properties for this fiber were 2.2 dB/Km at 1550 nm as amultimode attenuation and 0.28 dB/km at 1550 nm for the fundamentalmode.

Example 2

3000 grams of SiO2 (0.47 g/cc density) soot were flame deposited onto a1 meter long×8 mm diameter pure silica core cane. This assembly was thensintered as follows. The assembly was first dried for 2 hours in anatmosphere consisting of helium and 3 percent chlorine at 1000° C.followed by down driving at 32 mm/min through a hot zone set at 1500° C.in a 70 percent nitrogen and 30 percent helium (by volume) atmosphere,then re-down driven through the hot zone at 25 mm/min, then finalsintered at 6 mm/min, in order to sinter the soot to anitrogen/helium-seeded overclad blank. The blank was placed for 24 hoursin an argon purged holding oven set at 1000° C. to outgas the heliumfrom the blank.

The blank was drawn to 125 micron diameter fiber in a manner similar toExample 1. SEM analysis of the end face of a fiber showed about 22micron diameter solid silica core and a cladding containing 4.5 regionalvoid area percent nitrogen filled voids with an average diameter of 0.45microns and the smallest diameter holes at 0.03 microns and a maximumdiameter of 1.17 microns with a standard deviation of 0.19 microns, andcomprising approximately 2300 holes, resulting in about 8400 totalnumber of holes across the entire fiber cross-section. The total fibervoid area percent (area of the holes divided by total area of theoptical fiber cross-section×100) was about 4.4 percent. Opticalproperties for this fiber were 9.8 dB/Km at 1550 nm when measured as amultimode attenuation.

Example 3

3000 grams of SiO2 (0.46 g/cc density) soot were flame deposited onto a1 meter long×8 mm diameter pure silica core cane from Step 1. Thisassembly was then sintered as follows. The assembly was first dried for2 hours in an atmosphere consisting of 3 percent chlorine gas, theremainder helium, at 1000° C. followed by down driving at 32 mm/minthrough a hot zone set at 1500° C. in a 50 percent nitrogen/50 percenthelium (by volume) atmosphere. The assembly was then re-down driventhrough the same hot zone at 25 mm/min, after which the assembly wasre-down driven through the same hot zone for final sintering at 6mm/min) in order to sinter the soot to a nitrogen/helium-seeded overcladblank. The blank was placed for 24 hours in an argon purged holding ovenset at 1000° C. to outgas the helium from the preform blank.

The resultant optical fiber preform was drawn to 125 micron diameterfiber in a manner similar to Example 1. SEM analysis of the end face ofa fiber showed a 22 micron diameter solid silica core and a claddingcontaining 2.6 regional void area percent (nitrogen) with an averagediameter of 0.42 microns and the smallest diameter holes at 0.03 micronsand a maximum diameter of 0.80 microns with a standard deviation of 0.14microns, and comprising approximately 2300 holes, resulting in about5700 total number of holes in the fiber cross-section. The total fibervoid area percent (area of the holes divided by total area of theoptical fiber cross-section×100) was about 2.5 percent. Opticalproperties for this fiber were 11.9 dB/Km at 1550 nm when measured as amultimode attenuation.

Example 4

3000 grams of SiO2 (0.40 g/cc density) were flame deposited onto a 1meter long×8 mm diameter pure silica core cane from Step 1. Thisassembly was then sintered as follows. The assembly was first dried for2 hours in an atmosphere of He and 3 percent chlorine at 1000° C.,followed by down driving the assembly at 32 mm/min through a hot zoneset at 1500° C. in an atmosphere consisting of 30 percent nitrogen/70percent helium (by volume). The assembly was then re-down driven throughthe same hot zone and atmosphere at 25 mm/min, after which the assemblywas again driven through the same hot zone and atmosphere for finalsintering at 6 mm/min) in order to sinter the soot to anitrogen/helium-seeded overclad blank. The blank was then placed for 24hours in an argon purged holding oven set at 1000° C. to outgas thehelium from the blank.

The resultant optical fiber preform was drawn to a 125 micron diameteroptical fiber in a manner similar to Example 1. SEM analysis of the endface of a fiber showed a 22 micron diameter solid silica core and acladding containing 2.0 regional void area percent (nitrogen) with anaverage diameter of 0.37 microns and the smallest diameter holes at 0.03microns and a maximum diameter of 0.89 microns with a standard deviationof 0.13 microns, and comprising approximately 2100 holes, resulting inabout 8100 total number of holes in the fiber cross-section. The totalfiber void area percent (area of the holes divided by total area of theoptical fiber cross-section×100) was about 2.6 percent. Opticalproperties for this fiber were 4.4 dB/Km at 1550 nm when measured as amultimode attenuation.

Example 5

3000 grams of SiO2 (0.38 g/cc density) were flame deposited onto a 1meter long×8 mm diameter pure silica core cane from Step 1. Thisassembly was then sintered as follows. The assembly was first dried for2 hours in a helium and 3 percent chlorine atmosphere @1000° C. followedby down driving at 32 mm/min through a hot zone set at 1500° C. in a 15percent nitrogen/85 percent helium (by volume) atmosphere. The assemblywas then re-down driven through the same hot zone and sinteringatmosphere at 25 mm/min, after which the assembly was again driventhrough the same hot zone and sintering atmosphere for the finalsintering step at 6 mm/min in order to sinter the soot to anitrogen/helium-seeded overclad blank. The blank was then placed for 24hours in an argon purged holding oven set at 1000° C. to outgas thehelium from the blank.

The resultant optical fiber preform was drawn to 125 micron diameterfiber in a manner similar to Example 1. SEM analysis of the end face ofa fiber showed a 22 micron diameter solid silica core and a claddingcontaining 2.0 regional void area percent (nitrogen) with an averagediameter of 0.37 microns and the smallest diameter holes at 0.03microns. Optical properties for this fiber were 9.1 dB/Km at 1550 nmwhen measured as a multimode attenuation.

Example 6

3000 grams of SiO2 (0.5 g/cc density) were deposited onto a 1 meterlong×10 mm diameter removable alumina bait rod; after soot deposition,the alumina bait rod was removed. This assembly was then sintered asfollows. The assembly was first dried for 2 hours in an atmosphereconsisting of helium +3 percent chlorine at 1000° C., followed by downdriving at 32 mm/min through a hot zone set at 1500° C. in a 100 percentnitrogen atmosphere. The assembly was then re-down driven through thesame hot zone and atmosphere at 25 mm/min, after which the assembly wasagain down driven through the same hot zone and atmosphere for finalsintering at 6 mm/min in order to sinter the soot to anitrogen/helium-seeded overclad blank. The blank was then placed for 24hours in an argon purged holding oven set at 1000° C. to outgas thehelium. A 3 mm pure silica core cane from Step 1 was inserted into thecenterline of nitrogen-seeded SiO2 glass blank.

The resultant optical fiber preform was then drawn to 125 microndiameter fiber in a manner similar to Example 1 with <250 torr (vacuum)pulled on the centerline from the top of the blank to assure claddingmating to the core cane during the draw process. SEM analysis of the endface of a fiber showed 8 micron diameter solid silica core and acladding containing 4.0 regional void area percent (nitrogen) with anaverage diameter of 0.33 microns and the smallest diameter holes at 0.03microns and a maximum diameter of 0.82 microns with a standard deviationof 0.14 microns, and comprising approximately 4100 holes. Opticalproperties for this fiber showed that it was single moded at wavelengthsabove about 800 nm and attenuation was 4.8 and 4.5 dB/Km at 850 and 1550nm respectively, and a mode field diameter at 1550 nm of about 11microns. This fiber showed high bend resistance; it had very lowattenuation increase of only 2-8 dB at 1550 nm per turn when wrappedaround a 10 mm diameter mandrel (as compared to a standard commerciallyavailable SiO2-Ge02 0.35delta step index conventional single mode fiberwhich had approximately 25 dB attenuation delta per turn at 1550 nm forthe same radius of bending). This indicates that void containing cladfibers of the present invention can be made to exhibit less than 40,more preferably less than 30, even more preferably less than 20, andmost preferably less than 10 dB bend induced attenuation delta (i.e.,attenuation increase) per turn at 1550 nm (i.e., the attenuationmeasured on a straight length minus the attenuation measured on samelength of fiber wrapped around a mandrel) when wrapped around a 10 mmdiameter mandrel.

Example 7

3000 grams of SiO2 (0.5 g/cc density) were flame deposited onto a 1meter long×8 mm diameter cane having a step index with a small pedestal(0.39 percent delta step from 0 to 1.3 mm radius from center of cane,0.06 percent delta pedestal from 1.3 to 2.3 mm radius from center ofcane and pure silica from 2.3 to 4 mm radius from center of cane)GeO2-SiO2 core-pedestal with SiO2 clad cane made similar to the processused to make a cane from Step 1. This assembly was then sintered asfollows. The assembly was first percent held for 2 hours in a 100percent air atmosphere (˜78% N2+˜21% O2+˜1% A+˜0.03% CO2, by volume) at1000° C. followed by down driving the assembly at 6 mm/min through a hotzone set at 1500° C. in a 100 percent air atmosphere (˜78% N2+˜21%O2+˜1% Ar+˜0.03% CO2, by volume) in order to sinter the soot to aair-seeded (˜78% N2+˜21% O2+˜1% Ar+˜0.03% CO2, by volume) overcladblank. The blank was placed for 24 hours in an argon purged holding ovenset at 1000° C.

The resultant optical fiber preform was drawn to 125 micron diameterfiber in a manner similar to Example 1. SEM analysis of the end face ofa fiber showed an approximately 22 micron radius void-free solid corecane (containing the GeO2-SiO2 core as described in the above cane),surrounded by an approximately 39 micron outer radius void containingcladding region and a cladding ring of holes holes comprising 2.9regional void area percent (air (˜78% N2+˜21% O2+˜1% Ar+˜0.03% CO2, byvolume)) with an average diameter of 0.29 microns and the smallestdiameter holes at 0.03 microns and a maximum diameter of 1.4 microns,which is surrounded by a void-free pure silica outer cladding having anouter diameter of 125 microns (all radial dimensions measured from thecenter of the optical fiber).resulting in about 350 total number ofholes in the fiber cross-section. Because of the relatively slowdowndrive and sinter rate, the holes were located adjacent to the regioncorresponding to where the GeO₂-SiO₂ core-SiO₂ clad core cane was duringconsolidation and extending out from a radial distance from the fibercenterline of 22 microns to about 39 microns radial distance across thefiber cross-section. The total void area percent (area of the holesdivided by total area of the optical fiber cross-section×100) was about0.12 percent. Optical properties for this fiber were 2.94, 1.58 and 1.9dB/Km at 850, 1310 and 1550 nm, respectively, when measured as amultimode attenuation and, 0.42 and 0.29 dB/Km @1310 and 1550 nm,respectively, for the fundamental mode when spliced to a single modedfiber.

Example 8

The consolidated blank made in Example 2 was redrawn to 8 mm canes at1900 C. in a redraw furnace. An overclad of 750 grams of SiO2 (0.54 g/ccdensity) soot were flame deposited onto a 1 meter long×8 mm diametercladded core cane (i.e. a pure silica core, airline clad made via 70percent nitrogen +30 percent helium in Example 2). This assembly wasthen sintered as follows. The assembly was first dried for 2 hours in anatmosphere consisting of helium and 3 percent chlorine at 1000° C.followed by down driving at 6 mm/min through a hot zone set at 1500° C.in a 100 percent helium atmosphere. The blank was placed for 24 hours inan argon purged holding oven set at 1000° C. to outgas the helium fromthe blank. The overclad portion located outside of the hole containingcladding region was void free consolidated glass containing no holes.

The blank was drawn to 125 micron diameter fiber in a manner similar toExample 1. SEM analysis of the end face of a fiber showed about 4 micronradius solid silica core surrounded by an approximately 18 micron radiusairline-containing near clad region comprising 2.9 regional void areapercent (nitrogen) with an average diameter of 0.45 microns and thesmallest diameter holes at 0.03 microns and a maximum diameter of 1.26microns, a standard deviation of 0.19 microns, and comprisingapproximately 300 holes. The overclad portion located outside of theairline containing cladding region was void free consolidated glasscontaining no holes (all radial dimensions measured from the center. Thetotal fiber void area percent (area of the holes divided by total areaof the optical fiber cross-section×100) was about 3.4 percent. Themultimode attenuation for this fiber was 10.5 dB/Km at 1550 nm.

Example 9

7000 grams of SiO2 (0.5 g/cc density) were flame deposited onto a 1meter long×22 mm diameter step index (0.35 percent delta, 0.33 core/claddiameter ratio) GeO2-SiO2 core-SiO2 clad cane similar to the processused to make a cane from Step 1. This assembly was then sintered asfollows. The assembly was first dried for 2 hours in helium +3 percentChlorine at 1000° C. followed by down driving the assembly at 32 mm/minthrough a hot zone set at 1500° C. in a 2 percent CO and 98 percenthelium (by volume) atmosphere. The assembly was then re-down driventhrough the same hot zone and sintering atmosphere at 25 mm/min, afterwhich the assembly was again driven through the same hot zone andsintering atmosphere for the final sintering step at 6 mm/min in orderto sinter the soot to a CO/helium-seeded overclad blank. The blank wasplaced for 24 hours in an argon purged holding oven set at 1000° C.

The resultant optical fiber preform was drawn to 125 micron diameterfiber in a manner similar to Example 1. SEM analysis of the end face ofa fiber showed a 24 micron diameter solid core and inner cladding (8micron diameter GeO2-SiO2 core, 24 micron diameter SiO2 inner cladding)and an overcladding containing 1.8 regional void area percent (CO) withan average diameter of 0.41 microns and the smallest diameter holes at0.03 microns and a maximum diameter of 0.84 microns with a standarddeviation of 0.21 microns, and comprising approximately 1100 holes.Optical properties for this fiber were 1.95, 1.44 and 0.72 dB/Km at 850,1310 and 1550 nm, respectively, when measured as a multimode attenuationand 0.30 and 0.43 dB/Km at 1310 and 1550 nm, respectively, when splicedto a single moded fiber and measuring the fundamental mode for thisfiber.

Example 10

3000 grams of SiO2 (0.4 g/cc density) were deposited onto a 1 meterlong×10 mm diameter removable alumina bait rod; after soot deposition,the alumina bait rod was removed. This assembly was then sintered asfollows. The assembly was first dried for 2 hours in an atmosphereconsisting of helium plus 3 percent chlorine at 1000° C., followed bydown driving at 32 mm/min through a hot zone set at 1500° C. in a 100percent CF4 atmosphere. The assembly was then re-down driven through thesame hot zone and atmosphere at 25 mm/min, after which the assembly wasagain down driven through the same hot zone and atmosphere for finalsintering at 6 mm/min in order to sinter the soot to a CF4 (and/orCF4-gas reaction products with the silica including CO and CO2)-seededoverclad blank. The blank was then placed for 24 hours in an argonpurged holding oven set at 1000° C.

The resultant optical fiber preform was then drawn to 125 microndiameter fiber in a manner similar to Example 1 except back pressure of˜850 Torr positive pressure of nitrogen was kept on the centerline tokeep the center hole open. SEM analysis of the end face of a fibershowed a 125 micron fiber with a 28 micron diameter hole as the core anda cladding containing 2.8 regional void area percent (CF4/CO/CO2) withan average diameter of 0.67 microns and the smallest diameter holes at0.17 microns and a maximum diameter of 1.4 microns with a standarddeviation of 0.26 microns, and comprising approximately 700 holes.

Example 11

The consolidated blank made in Example 2 was redrawn to 8 mm canes at1900 C. in a redraw furnace. An overclad of 750 grams of SiO2 (0.56 g/ccdensity) soot were flame deposited onto a 1 meter long×8 mm diameterpure silica core, airline clad (made via 100 percent nitrogen in Example17) cane. This assembly was then sintered as follows. The assembly wasfirst dried for 2 hours in an atmosphere consisting of helium and 3percent chlorine at 1000° C. followed by down driving at 32 mm/minthrough a hot zone set at 1500° C. in a 100 percent nitrogen (by volume)atmosphere, then re-down driven through the hot zone at 25 mm/min, thenfinal sintered at 6 mm/min, in order to sinter the soot to anitrogen/helium-seeded overclad blank. The blank was placed for 24 hoursin an argon purged holding oven set at 1000° C. to outgas the heliumfrom the blank.

The blank was drawn to 125 micron diameter fiber in a manner similar toExample 1. Scanning Electron Microscope image analysis of the end faceof a fiber showed about 4 micron radius solid silica core regionsurrounded by an approximately 16 micron outer radius void-containingnear clad region containing approximately 11.6 volume percent holes(nitrogen) with a mean hole diameter of 0.70 microns, which issurrounded by a void-containing silica outer cladding having an outerdiameter of 125 microns (all radial dimensions measured from the centerof the optical fiber) containing 4.7 volume percent holes (nitrogen)with a mean hole diameter of 0.54 microns, the smallest diameter holesat 0.03 microns, a maximum diameter of 0.87 microns with a standarddeviation of 0.23 microns. Thus demonstrating that different levels ofvoids with respect to fiber radius and therefore an index profile ofdifferent levels of differing percent voids can be achieved. Opticalproperties for this fiber were 17.4 dB/Km at 1550 nm when measured as amultimode attenuation.

Example 12

500 grams of SiO2 (0.46 g/cc density) soot were flame deposited onto a 1meter long×15 mm diameter pure silica core cane. This assembly was thensintered as follows. The assembly was first dried for 2 hours in anatmosphere consisting of helium and 3 percent chlorine at 1000° C.followed by down driving at 32 mm/min through a hot zone set at 1500° C.in a 70 percent nitrogen and 30 percent SiF4 (by volume) atmosphere,then re-down driven through the hot zone at 25 mm/min in the sameatmosphere, then final sintered in 100 percent nitrogen at 6 mm/min, inorder to sinter the soot to a F-doped+nitrogen-seeded overclad blank.The blank was placed for 24 hours in an argon purged holding oven set at1000° C.

The blank was drawn to 125 micron diameter fiber in a manner similar toExample 1. Optical image analysis at 200 and 500 fold magnification ofthe end face of a fiber showed about 82 micron diameter solid silicacore and a cladding containing approximately 9.0 volume percent holes(nitrogen) with a mean hole diameter of 0.73 microns diameter thesmallest diameter holes at 0.03 microns, a maximum diameter of 2.0microns with a standard deviation 0.40 microns, and comprisingapproximately 1200 holes. Optical properties for this fiber were 16.1,14.5 and 13.2 dB/Km at 850, 1310 and 1550 nm, respectively, whenmeasured as a multimode attenuation. Optical bend performance datashowed a 1.85 and 0.67 dB increase in attenuation at 850 and 1550 nm,respectively, when the fiber was wrapped once around a mandrel having a5 mm radius. The control fiber without voids was made; SiF4+He sinteringatmosphere was used in the cladding and resulted in a fiber with novoids. Optical bend performance of this control fiber showed a 8.06 and9.33 dB increase in attenuation at 850 and 1550 nm, respectively, whenthe fiber was wrapped once around a mandrel having a 5 mm radius. Theseresults demonstrate the superior bend performance of a fiber containingvoids in the cladding.

Example 13

500 grams of SiO2 (0.53 g/cc density) soot were flame deposited onto a 1meter long×15 mm diameter solid glass cane of GeO2-SiO2 graded index (2percent delta index (vs. silica) at peak with a parabolic shape). Thisassembly was then sintered as follows. The assembly was first dried for2 hours in an atmosphere consisting of helium and 3 percent chlorine at1000° C. followed by down driving at 32 mm/min through a hot zone set at1500° C. in a 100 percent nitrogen atmosphere, then re-down driventhrough the hot zone at 25 mm/min in the same atmosphere, then finalsintered in 100 percent nitrogen at 6 mm/min, in order to sinter thesoot to a nitrogen-seeded overclad blank. The blank was placed for 24hours in an argon purged holding oven set at 1000° C.

The blank was drawn to 125 micron diameter fiber in a manner similar toExample 1. Optical image analysis at 200 and 500 fold magnification ofthe end face of a fiber showed about 81 micron diameter solid germaniadoped silica core and a cladding containing approximately 3.5 volumepercent holes (nitrogen) with a mean hole diameter of 0.46 micronsdiameter the smallest diameter holes at 0.04 microns, a maximum diameterof 0.97 microns with a standard deviation of 0.16 microns, andcomprising approximately 1500 holes. Optical properties for this fiberwere 3.36, 1.09 and 0.84 dB/Km at 850, 1310 and 1550 nm, respectively,when measured as a multimode attenuation. Optical bend performance datashowed a less than 0.70 dB and 0.55 dB increase in attenuation at 850and 1550 nm, respectively, when the fiber was wrapped once around amandrel having a 5 mm radius. The commercially available 62.5 microncore (GeO2-SiO2 graded index (2 percent delta index (vs. silica) at peakwith a parabolic shape), 125 micron diameter control fiber without voidswas measured for bend resistance. Optical bend performance of thiscontrol fiber showed a 1.13 and 1.20 dB increase in attenuation at 850and 1550 nm, respectively, when the fiber was wrapped once around amandrel having a 5 mm radius. These results demonstrate the superiorbend performance of a fiber containing voids in the cladding.

Example 14

1200 grams of SiO2 (0.47 g/cc density) soot were flame deposited onto a1 meter long×15 mm diameter solid glass cane of GeO2-SiO2 graded index(2 percent delta index (vs. silica) at peak with a parabolic shape).This assembly was then sintered as follows. The assembly was first driedfor 2 hours in an atmosphere consisting of helium and 3 percent chlorineat 1000° C. followed by down driving at 32 mm/min through a hot zone setat 1500° C. in a 100 percent oxygen atmosphere, then re-down driventhrough the hot zone at 25 mm/min in the same atmosphere, then finalsintered in 100 percent oxygen at 6 mm/min, in order to sinter the sootto a oxygen-seeded overclad blank. The blank was placed for 24 hours inan argon purged holding oven set at 1000° C.

The blank was drawn to 125 micron diameter fiber in a manner similar toExample 1. Optical image analysis at 200 and 500 fold magnification ofthe end face of a fiber showed a 62.5 micron diameter solidsilica-germania core and a cladding containing approximately 9.0 volumepercent holes (oxygen) with a mean hole diameter of 0.45 micronsdiameter the smallest diameter holes at 0.03 microns, a maximum diameterof 1.2 microns with a standard deviation of 0.21 microns, and comprisingapproximately 400 holes. Measured attenuation for this fiber was 3.00,0.74 and 0.45 dB/Km at 850, 1310 and 1550 nm, respectively, whenmeasured as a multimode attenuation. Optical bend performance datashowed a less than 0.03 dB and less than 0.01 dB increase in attenuationat 850 and 1550 nm, respectively, when the fiber was wrapped once arounda mandrel having a 5 mm radius. The commercially available 62.5 microncore (GeO2-SiO2 graded index (2 percent delta index (vs. silica) at peakwith a parabolic shape), 125 micron diameter control fiber without voidswas measured for bend resistance. Optical bend performance of thiscontrol fiber showed a 1.13 and 1.20 dB increase in attenuation at 850and 1550 nm, respectively, when the fiber was wrapped once around amandrel having a 5 mm radius. These results demonstrate the superiorbend performance of a fiber containing voids in the cladding. Bandwidthmeasurements (overfill launch) were at 850 nm=200 MHz*km and at 1300nm=500 MHz*km. This example illustrates a microstructured optical fiberwhich is multimoded at 1550 nm. The fiber exhibits a core region havinga first refractive index and a cladding region having a secondrefractive index which is lower than that of the core region such thatthe light which is to be transmitted through the fiber is retainedgenerally within the core, wherein the cladding comprises at least oneregion in the cladding which is comprised of a plurality ofnon-periodically located voids. Such fibers preferably are multimoded at1550 nm and exhibit an increase in attenuation at 1550 nm, when saidfiber is wrapped once around a mandrel having a radius of 5 mm, which isless than 1 dB/km, more preferably less than 0.75, and most preferablyless than .05 db/km.

Example 15

The optical fiber preform described in example 8 was drawn to 125 microndiameter fiber at 3 meters/second in a furnace having an 8″ long hotzone set at 2000° C. SEM analysis of the end face of a fiber showedabout a 4 micron radius solid silica core surrounded by an approximately18 micron radius airline-containing near clad region containing 8.5regional void area percent (nitrogen filled) with a mean hole diameterof 0.63 microns diameter the smallest diameter holes at 0.03 microns, amaximum diameter of 1.9 microns, and a standard deviation of 0.32microns, which is surrounded by a void-free pure silica outer claddinghaving an outer diameter of 125 microns (all radial dimensions measuredfrom the center of the optical fiber). The fiber drawn in Example 8 hasonly 2.9 regional void area percent (nitrogen) with an average diameterof 0.45 microns; thus demonstrating that draw conditions (in this case alonger hot zone and faster draw speed) can be used to control the holeair-fill fraction and hole diameter. The overclad portion locatedoutside of the airline containing cladding region was void freeconsolidated glass containing no holes.

Example 16

3000 grams of SiO2 (0.53 g/cc density) soot were flame deposited onto a1 meter long×8 mm diameter pure silica core cane. This assembly was thensintered as follows. The assembly was first dried for 2 hours in anatmosphere consisting of helium and 3 percent chlorine at 1000° C.followed by down driving at 32 mm/min through a hot zone set at 1500° C.in a 100% argon (by volume) atmosphere, then re-down driven through thehot zone at 25mm/min, then final sintered in argon at 6 mm/min, in orderto sinter the soot to an argon-seeded overclad blank. The blank wasplaced for 24 hours in an argon purged holding oven set at 1000° C. tooutgas the helium from the blank. The blank was drawn to 125 microndiameter fiber in a manner similar to Example 1. SEM analysis of the endface of a fiber showed about 22 micron diameter solid silica core and acladding containing approximately 8.0 regional void area percent (argon)with a mean hole diameter of 0.35 microns diameter the smallest diameterholes at 0.03 microns, a maximum diameter of 0.85 microns, and astandard deviation of 0.15 microns. Optical properties for this fiberwere 1.65 and 1.20dB/Km at 1310 and 1550 nm, respectively, when measuredas a multimode attenuation.

Example 17

3000 grams of SiO2 (0.55 g/cc density) soot were flame deposited onto a1 meter long×8 mm diameter pure silica core cane. This assembly was thensintered as follows. The assembly was first dried for 2 hours in anatmosphere consisting of helium and 3 percent chlorine at 1000° C.followed by down driving at 32 mm/min through a hot zone set at 1500° C.in a 100% nitrogen (by volume) atmosphere, then re-down driven throughthe hot zone at 25mm/min, then final sintered at 6 mm/min, in order tosinter the soot to an nitrogen-seeded overclad blank. The blank wasplaced for 24 hours in an argon purged holding oven set at 1000° C. tooutgas the helium from the blank. The blank was drawn to 125 microndiameter fiber in a manner similar to Example 1. SEM analysis of the endface of a fiber showed about 22 micron diameter solid silica core and acladding containing 2.0 regional void area percent (nitrogen) with anaverage diameter of 0.22 microns, the smallest diameter holes at 0.03microns and the largest diameter of 0.50 microns, and a standarddeviation of 0.08 microns. Optical properties for this fiber were 1.28and 0.87 dB/Km at 1310 and 1550 nm, respectively, when measured as amultimode attenuation and 0.28 dB/Km at 1550 nm when spliced to a singlemoded fiber and measuring the fundamental mode for this fiber.

Example 18

4600 grams of SiO2 (0.42 g/cc density) soot were flame deposited onto a1 meter long×10 mm diameter step index (0.35 percent delta, 0.33core/clad diameter ratio) GeO2-SiO2core-SiO2 clad cane similar to theprocess used to make a cane from Step 1. This assembly was then sinteredas follows. The assembly was first dried for 2 hours in an atmosphereconsisting of helium and 3 percent chlorine at 1000° C. followed by downdriving at 6 mm/min through a hot zone set at 1500° C. in a 100% oxygen(by volume) atmosphere in order to sinter the soot to an oxygen-seededoverclad blank. The blank was placed for 24 hours in an argon purgedholding oven set at 1000° C. to outgas the helium from the blank.

The optical fiber preform was drawn to 125 micron diameter fiber at 18meters/second in a furnace having an 8″ long hot zone set at 2000° C.The blank was drawn to 125 micron diameter fiber in a maimer similar toExample 15. SEM analysis of the end face of the fiber showed anapproximately 4 micron radius GeO₂-SiO₂ center core region surrounded byan approximately 12 micron outer radius void-free near clad regionsurrounded by an approximately 18 micron outer radius void containingcladding region which is surrounded by a void-free pure silica outercladding having an outer diameter of 125 microns (all radial dimensionsmeasured from the center of the optical fiber). The void containing ringregion comprised 4.2 percent regional area percent holes (100 percent O₂by volume) in that area with an average diameter of 0.53 microns and thesmallest diameter holes at 0.18 microns and a maximum diameter of 1.4microns, resulting in about 85 total number of holes in the fibercross-section. Because of the relatively slow downdrive and sinter rate,the holes were located adjacent to the region corresponding to where theGeO₂-SiO₂ core-SiO₂ clad core cane was during consolidation andextending out from a radial distance from the fiber centerline of 12microns to about 18 microns radial distance across the fibercross-section. The total fiber void area percent (area of the holesdivided by total area of the optical fiber cross-section×100) was about0.21 percent. Optical properties for this fiber were 0.34 and 0.21 dB/Kmat 1310 and 1550 nm, respectively, and a fiber cutoff showing the fiberwas single moded above 1230 nm, thereby making the fiber single moded atwavelengths above 1230 nm. A portion of this fiber was measured for bendperformance around a 10 mm diameter mandrel, and the fiber exhibited anincrease in attenuation at 1550 nm of about 0.7 dB/turn, thusdemonstrating that attenuation increases of even less than 5 dB/turnaround a 10 mm diameter mandrel are achievable using the methodsdisclosed herein. This same portion of the fiber was measured for bendperformance around a 20 mm diameter mandrel, and the fiber exhibited anincrease in attenuation at 1550 nm of about 0.08 dB/turn, thusdemonstrating that attenuation increases of less than 1 dB/turn, andmore preferably less than 0.5 dB/turn around a 20 mm diameter mandrelare achievable using the methods disclosed herein.

Example 19

290 grams of SiO2 (0.47 g/cc density) soot were deposited via OVD onto afully consolidated 1 meter long×10.4 mm diameter step index (0.35percent delta, 0.33 core/clad diameter ratio) GeO2-SiO2 core-SiO2 cladcore cane, thereby resulting in a preform comprising a consolidated coreregion which was surrounded by a consolidated silica cladding regionwhich in turn was surrounded by a soot silica region. The soot claddingof this assembly was then sintered as follows. The assembly was firstdried for 2 hours in an atmosphere consisting of helium and 3 percentchlorine at 1000° C. followed by down driving at 200 mm/min(corresponding to approximately a 100° C./min temperature increase forthe outside of the soot preform during the downdrive process) through ahot zone set at 1490° C. in a 100 percent oxygen sintering atmosphere.The preform assembly was then re-down driven (i.e., a second time)through the hot zone at 100 mm/min (corresponding to approximately a 50°C./min temperature increase for the outside of the soot preform duringthe downdrive process). The preform assembly was then re-down driven(i.e., a third time) through the hot zone at 50 mm/min (corresponding toapproximately a 25° C./min temperature increase for the outside of thesoot preform during the downdrive process). The preform assembly wasthen re-down driven (i.e., a forth time) through the hot zone at 25mm/min (corresponding to approximately a 12.5° C./min temperatureincrease for the outside of the soot preform during the downdriveprocess), then final sintered at 6 mm/min (approximately 3° C./mm heatup rate) in order to sinter the soot into a oxygen-seeded overcladblank. The first series of higher downfeed rate were employed to glazethe outside of the optical fiber preform, which facilitates trapping ofthe gases in the preform. The blank was then placed for 24 hours in anargon purged holding oven set at 1000° C. This preform was then placedback in a lathe where 3600 grams of additional SiO2 (0.42 g/cc density)soot were deposited via OVD. The soot of this cladding (which may becalled overcladding) for this assembly was then sintered as follows. Theassembly was first dried for 2 hours in an atmosphere consisting ofhelium and 3 percent chlorine at 1000° C. followed by down driving at 6mm/min through a hot zone set at 1500° C. in a 100% helium (by volume)atmosphere in order to sinter the soot to an Germania containingvoid-free core, silica void-free inner cladding, silica oxygen-seededring (i.e., silica with holes containing oxygen), and void-free overcladblank. The blank was placed for 24 hours in an argon purged holding ovenset at 1000° C. to outgas the helium from the blank. The optical fiberpreform was drawn to about 125 micron diameter fiber at 20 meters/secondin a furnace having an 8″ long hot zone set at 2000° C. SEM analysis ofthe end face of a fiber showed an approximately 4 micron radiusGeO2-SiO2 core surrounded by a 12 micron outer radius void-free nearclad region surrounded by 18 micron outer radius void containingcladding region (ring thickness of approximately 6 microns) which issurrounded by a void-free pure silica outer cladding having an outerdiameter of about 125 microns (all radial dimensions measured from thecenter of the optical fiber). The void containing ring region comprised2.7 percent regional area percent holes (100 percent O2 by volume) inthat area with an average diameter of 0.36 microns and the smallestdiameter holes at 0.05 microns and a maximum diameter of 0.8 microns,resulting in about 105 total number of holes in the fiber cross-section.The total fiber void area percent (area of the holes divided by totalarea of the optical fiber cross-section×100) was about 0.1 percent.Optical properties for this fiber were 0.33 and 0.19 dB/Km at 1310 and1550 nm, respectively, and a fiber cutoff of about 1250 nm, therebymaking the fiber single moded at wavelengths above 1250 nm. A portion ofthis fiber was measured for bend performance around a 10 mm diametermandrel, and the fiber exhibited an increase in attenuation at 1550 nmof about 0.2 dB/turn, thus demonstrating attenuation increases of evenless than 1 dB/turn, preferably less than 0.5 dB/turn, around a 10 mmdiameter mandrel. This same portion of the fiber was measured for bendperformance around a 20 mm diameter mandrel, and the fiber exhibited anincrease in attenuation at 1550 nm of about 0.02 dB/turn, thusdemonstrating that attenuation increases of less than 1 dB/turn, andmore preferably less than 0.1 dB/turn, and still more preferably lessthan 0.05 dB/turn, around a 20 mm diameter mandrel are achievable. Thissame portion of the fiber was measured for bend performance around a 6mm diameter mandrel, and the fiber exhibited an increase in attenuationat 1550 nm of about 2 dB/turn, thus demonstrating that attenuationincreases of less than 10 dB/turn, and more preferably less than 5dB/turn, and still more preferably less than 3 dB/turn, around a 6 mmdiameter mandrel are achievable.

Example 20

450 grams of SiO2 (0.37 g/cc density) soot were deposited via OVD onto afully consolidated 1 meter long×22 mm diameter step index (0.35 percentdelta, 0.33 core/clad diameter ratio) GeO2-SiO2 core-SiO2 clad corecane, thereby resulting in a preform comprising a consolidated coreregion which was surrounded by a consolidated silica cladding regionwhich in turn was surrounded by a soot silica region. The soot claddingof this assembly was then sintered as follows. The assembly was firstdried for 2 hours in an atmosphere consisting of helium and 3 percentchlorine at 1000° C. followed by down driving at 200 mm/min(corresponding to approximately a 100° C./min temperature increase forthe outside of the soot preform during the downdrive process) through ahot zone set at 1490° C. in a 100 percent nitrogen sintering atmosphere.The preform assembly was then re-down driven (i.e., a second time)through the hot zone at 100 mm/min (corresponding to approximately a 50°C./min temperature increase for the outside of the soot preform duringthe downdrive process). The preform assembly was then re-down driven(i.e., a third time) through the hot zone at 50 mm/min (corresponding toapproximately a 25° C./min temperature increase for the outside of thesoot preform during the downdrive process). The preform assembly wasthen re-down driven (i.e., a forth time) through the hot zone at 25mm/min (corresponding to approximately a 12.5° C./min temperatureincrease for the outside of the soot preform during the downdriveprocess), then final sintered at 6 mm/min (approximately 3° C./min heatup rate) in order to sinter the soot into a nitrogen-seeded overcladblank. The first series of higher downfeed rate were employed to glazethe outside of the optical fiber preform, which facilitates trapping ofthe gases in the preform. The blank was then placed for 24 hours in anargon purged holding oven set at 1000° C. The blank was then redrawn at1900° C. in a redraw furnace into 13 mm diameter canes. A 1 meter long13 mm diameter cane from the previous step was then placed back in alathe where 4700 grams of additional SiO2 (0.37 g/cc density) soot weredeposited via OVD. The soot of this cladding (which maybe calledovercladding) for this assembly was then sintered as follows. Theassembly was first dried for 2 hours in an atmosphere consisting ofhelium and 3 percent chlorine at 1000° C. followed by down driving at 6mm/min through a hot zone set at 1500° C. in a 100% helium (by volume)atmosphere in order to sinter the soot to an Germania containingvoid-free core, silica void-free inner cladding, silica nitrogen-seededring (i.e., silica with holes containing nitrogen), and void-freeoverclad blank. The blank was placed for 24 hours in an argon purgedholding oven set at 1000° C. to outgas the helium from the blank. Theoptical fiber preform was drawn to about 125 micron diameter fiber at 10meters/second in a furnace having an 8″ long hot zone set at 2000 ° C.Optical microscope imaging of the end face of a fiber showed anapproximately 4 micron radius GeO2-SiO2 core surrounded by a 12 micronouter radius void-free near clad region surrounded by 15 micron outerradius void containing cladding region (ring thickness of approximately3 microns) which is surrounded by a void-free pure silica outer claddinghaving an outer diameter of about 125 microns (all radial dimensionsmeasured from the center of the optical fiber). The void containing ringregion comprised approximately 3 percent regional area percent holes(100 percent N2 by volume) in that area with an average diameter ofapproximately 0.2 microns. The total fiber void area percent (area ofthe holes divided by total area of the optical fiber cross-section×100)was about 0.1 percent. Optical properties for this fiber were 0.34 and0.196 dB/Km at 1310 and 1550 nm, respectively, and a fiber cutoff ofabout 1290 nm, thereby making the fiber single moded at wavelengthsabove 1290 nm. A portion of this fiber was measured for bend performancearound a 10 mm diameter mandrel, and the fiber exhibited an increase inattenuation at 1550 nm of about 0.11 dB/turn, thus demonstratingattenuation increases of even less than 1 dB/turn, preferably less than0.5 dB/turn, around a 10 mm diameter mandrel. This same portion of thefiber was measured for bend performance around a 20 mm diameter mandrel,and the fiber exhibited an increase in attenuation at 1550 nm of about0.016 dB/turn, thus demonstrating that attenuation increases of lessthan 1 dB/turn, and more preferably less than 0.1 dB/turn, and stillmore preferably less than 0.05 dB/turn, around a 20 mm diameter mandrelare achievable.

Example 21

130 grams of SiO2 (0.37 g/cc density) soot were deposited via OVD onto afully consolidated 1 meter long×10.5 mm diameter step index (0.35percent delta, 0.33 core/clad diameter ratio) GeO2-SiO2 core-SiO2 cladcore cane, thereby resulting in a preform comprising a consolidated coreregion which was surrounded by a consolidated silica cladding regionwhich in turn was surrounded by a soot silica region. The soot claddingof this assembly was then sintered as follows. The assembly was firstdried for 2 hours in an atmosphere consisting of helium and 3 percentchlorine at 1000° C. followed by down driving at 200 mm/min(corresponding to approximately a 100° C./min temperature increase forthe outside of the soot preform during the downdrive process) through ahot zone set at 1490° C. in a 100 percent argon sintering atmosphere.The preform assembly was then re-down driven (i.e., a second time)through the hot zone at 100 mm/min (corresponding to approximately a 50°C./min temperature increase for the outside of the soot preform duringthe downdrive process). The preform assembly was then re-down driven(i.e., a third time) through the hot zone at 50 mm/min (corresponding toapproximately a 25° C./min temperature increase for the outside of thesoot preform during the downdrive process). The preform assembly wasthen re-down driven (i.e., a forth time) through the hot zone at 25mm/min (corresponding to approximately a 12.5° C./min temperatureincrease for the outside of the soot preform during the downdriveprocess), then final sintered at 6 mm/min (approximately 3° C./min heatup rate) in order to sinter the soot into a argon-seeded overclad blank.The first series of higher downfeed rate were employed to glaze theoutside of the optical fiber preform, which facilitates trapping of thegases in the preform. The blank was then placed for 24 hours in an argonpurged holding oven set at 1000° C. This preform was then placed back ina lathe where 5000 grams of additional SiO2 (0.44 g/cc density) sootwere deposited via OVD. The soot of this cladding (which may be calledovercladding) for this assembly was then sintered as follows. Theassembly was first dried for 2 hours in an atmosphere consisting ofhelium and 3 percent chlorine at 1000° C. followed by down driving at 6mm/min through a hot zone set at 1500° C. in a 100% helium (by volume)atmosphere in order to sinter the soot to an Germania containingvoid-free core, silica void-free inner cladding, silica argon-seededring (i.e., silica with holes containing argon), and void-free overcladblank. The blank was placed for 24 hours in an argon purged holding ovenset at 1000° C. to outgas the helium from the blank. The optical fiberpreform was drawn to about 125 micron diameter fiber at 20 meters/secondin a furnace having an 8″ long hot zone set at 2000° C. Opticalmicroscope imaging of the end face of a fiber showed an approximately 4micron radius GeO2-SiO2 core surrounded by a 12 micron outer radiusvoid-free near clad region surrounded by 16 micron outer radius voidcontaining cladding region (ring thickness of approximately 4 microns)which is surrounded by a void-free pure silica outer cladding having anouter diameter of about 125 microns (all radial dimensions measured fromthe center of the optical fiber). The void containing ring regioncomprised of argon having in that area with an average diameter ofapproximately 0.3 microns. Optical properties for this fiber were 0.37and 0.226 dB/Km at 1310 and 1550 nm, respectively, and a fiber cutoff ofabout 1270 nm, thereby making the fiber single moded at wavelengthsabove 1270 nm. A portion of this fiber was measured for bend performancearound a 10 mm diameter mandrel, and the fiber exhibited an increase inattenuation at 1550 nm of about 0.27 dB/turn, thus demonstratingattenuation increases of even less than 1 dB/turn, preferably less than0.5 dB/turn, around a 10 mm diameter mandrel. This same portion of thefiber was measured for bend performance around a 20 mm diameter mandrel,and the fiber exhibited an increase in attenuation at 1550 nm of about0.026 dB/turn, thus demonstrating that attenuation increases of lessthan 1 dB/turn, and more preferably less than 0.1 dB/turn, and stillmore preferably less than 0.05 dB/turn, around a 20 mm diameter mandrelare achievable.

Comparative Example

A blank was made similar to Example 1 but sintered in helium only. Theassembly was first dried for 2 hours in an atmosphere of helium and 3percent chlorine at 1000° C., followed by down driving the assembly at32 mm/min through a hot zone set at 1500° C. in an atmosphere consistingof 100 percent helium. The assembly was then re-down driven through thesame hot zone and atmosphere at 25 mm/min, after which the assembly wasagain driven through the same hot zone and atmosphere for finalsintering at 6 mm/min). As was expected, the clad glass was found tocontain no seeds. The blank was placed for 24 hours in an argon purgedholding oven set at 1000° C. to outgas the helium. When the blank wasdrawn to 125 micron fiber in a manner similar to Example 1, it was foundto contain no holes (this was expected). A 2.4 Km length of fiber didnot transmit light as measured by the cutback method (indicating thatthe attenuation was greater than 100 dB/Km); this was expected sincethere was no refractive index contrast between the core and the clad.

Comparative Example

A blank was made similar to Example 1 but sintered in helium only. Theassembly was first dried for 2 hours in an atmosphere of helium and 3percent chlorine at 1000° C., followed by down driving the assembly at32 mm/min through a hot zone set at 1500° C. in an atmosphere consistingof 100 percent helium. The assembly was then re-down driven through thesame hot zone and atmosphere at 25 mm/min, after which the assembly wasagain driven through the same hot zone and atmosphere for finalsintering at 6 mm/min). As was expected, the clad glass was found tocontain no seeds. The blank was placed for 24 hours in an argon purgedholding oven set at 1000° C. to outgas the helium. When the blank wasdrawn to 125 micron fiber in a manner similar to Example 1, it was foundto contain no holes (this was expected). A 2.4 Km length of fiber didnot transmit light as measured by the cutback method (indicating thatthe attenuation was greater than 100 dB/Km); this was expected sincethere was no refractive index contrast between the core and the clad.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A method of making an optical fiber comprising forming via a CVDoperation a soot containing optical fiber preform; consolidating thesoot in said soot containing preform in a gaseous atmosphere underconditions which are effective to trap a portion of said gaseousatmosphere in said preform during said consolidation step, therebyforming a consolidated preform having voids in said preform, and usingsaid preform in a manufacturing process to make an optical fiber havinga core region having a first refractive index and a cladding regionhaving a second refractive index lower than that of the core, saidcladding comprising a region of non-periodically located voidssurrounding said core having a total fiber void area percent, when thefiber is viewed in cross-section, that is greater than 0.05 percent,wherein said consolidating step comprises first exposing said preform toa temperature greater than about 1500 C and a feed rate into aconsolidation furnace sufficient to result in at least a portion of saidpreform increasing in temperature at a rate greater than about 12 C/min.2. The method of claim 1, wherein said consolidating step comprisesconsolidating said soot containing perform in a furnace a temperaturegreater than 1500 C and heating said preform at at least a firsttemperature ramp rate greater than 10 C/minute, and said gaseousatmosphere comprises at least one gas selected from the group consistingof nitrogen, argon, CO₂, oxygen, chlorine, CF4, CO, SO2 and mixturesthereof, and wherein said using said preform in a manufacturing processstep comprises drawing an optical fiber from said perform such that saidvoids formed in consolidating step are retained in said fiber.
 3. Themethod of claim 2, wherein said soot preform comprises a tubularpreform, and said consolidated preform comprises a tubular preform. 4.The method of claim 1, wherein said soot containing preform in saidconsolidating step comprises at least a void free core region onto whichadditional soot has been deposited via said CVD operation, and saidconsolidating step results in said preform having voids in the claddingregion of said consolidated preform.
 5. The method of claim 1, whereinthe maximum cross sectional diameter of each of said voids is less than1550 nm.
 6. The method of claim 1, wherein said forming step comprisesdepositing soot via CVD onto the outside of a glass core rod and saidconsolidating step takes place after said forming step.
 7. The method ofclaim 1, wherein said using said perform step further comprises drawingsaid consolidated preform into an optical fiber, wherein said voidswhich were formed during said consolidation step are retained anddeformed during said drawing step and remain in the fiber after saiddrawing step.
 8. The method of claim 1, wherein said gaseous atmospherecomprises at least one gas selected from the group consisting of argon,nitrogen, carbon monoxide, carbon dioxide, oxygen, CF4, C2F6, Kr, andmixtures thereof.
 9. A method of making an optical fiber comprisingforming via a CVD operation a soot containing optical fiber preform;consolidating the soot preform in a gaseous atmosphere which comprisesat least one gas selected from the group consisting of nitrogen, argon,CO₂, oxygen, chlorine, CF4, CO, SO2 and mixtures thereof underconditions which are effective to trap a portion of said gaseousatmosphere in said preform during said consolidation step, therebyresulting in the formation of voids in said consolidated preform, andusing said consolidated preform in a manufacturing process to form anoptical fiber having a core region having a first refractive index and acladding region having a second refractive index lower than that of thecore such that light which is transmitted through the fiber is retainedgenerally within the core, whereby said voids are located at leastsubstantially in the cladding of said optical fiber, wherein said voidshave a total fiber void area percent, when the fiber if viewed incross-section, that is greater than 0.05 percent, and wherein saidconsolidating step comprises first exposing said preform to atemperature greater than about 1500 C and a feed rate into aconsolidation furnace sufficient to result in at least a portion of saidpreform increasing in temperature at a rate greater than about 12 C/min.10. The method of claim 9, wherein said consolidating step comprisesfirst exposing said preform to a temperature and feed rate into aconsolidation furnace sufficient to result in said preform increasing intemperature at a rate greater than about 14° C./min.
 11. The method ofclaim 10, wherein said consolidating step comprises exposing the preformin a furnace which is maintained at a temperature of equal to or greaterthan 1390° C.
 12. The method of claim 9, wherein said consolidating stepfurther comprises, subsequent to said first exposing step, exposing saidpreform to a temperature and feed rate into a consolidation furnacesufficient to result in said preform increasing in temperature at a ratewhich is at least 2° C./min less than in said first exposing step. 13.The method of claim 12, wherein said atmosphere comprises at least onegas selected from the group consisting of nitrogen, argon andcombinations thereof in an amount greater than 85 percent by volume, theremainder of said atmosphere at least substantially made up of helium.14. The method of claim 9, wherein said atmosphere comprises nitrogen orargon in an amount greater than 65 percent by volume.
 15. The method ofclaim 9, said method further comprising either (1) forming said sootcontaining preform by depositing said soot onto a core cane doped withgermanium or (2) inserting a germanium doped core cane into a voidcontaining cladding region formed via said consolidating step.
 16. Themethod of claim 9, wherein said region containing non-periodicallydistributed holes is not immediately adjacent to the core.